The invention relates to production of high-purity hydrogen by a combination of membrane gas separation, steam reforming and pressure swing adsorption.
A number of off-gas streams containing hydrogen and hydrocarbons are generated during refinery and petrochemical plant operations. These streams include overheads from phase separators; fractionation columns; stabilization columns; demethanizers; debutanizers; absorption, stripping and scrubbing units; and so on. In some cases, the composition of the stream renders it suitable for reintroduction into the train of operations upstream or downstream of its generation point. Frequently, however, the stream composition is such that it is not cost-effective to treat it further and it is passed to the plant fuel header.
Streams passed to the fuel header represent lost products, both hydrogen and hydrocarbons; in addition, only a finite quantity of fuel gas is needed, so plants can become bottlenecked by over supply of fuel gas. Meanwhile, most refineries operate with a hydrogen deficit, and the demands of the refining and chemical industries for high-purity hydrogen continue to grow year by year. Improved processes for hydrogen manufacture and/or recovery from such light hydrocarbon/hydrogen off-gases would clearly be useful to industry.
For hydrogen separation from light hydrocarbons, techniques that have been employed in refineries and petrochemical plants include pressure swing adsorption (PSA) and membrane separation. Representative references that teach the use of PSA to treat off-gases from petrochemical processes include U.S. Pat. Nos. 5,332,492 and 5,457,256, to UOP, and U.S. Pat. No. 5,675,052, to BOC. Representative references describing membrane separation processes include U.S. Pat. Nos. 4,362,613 and 4,367,135, to Monsanto, U.S. Pat. No. 4,548,619, to UOP, U.S. Pat. No. 5,053,067, to L""Air Liquide, U.S. Pat. No. 5,082,481, to Lummus Crest, U.S. Pat. No. 5,157,200, to Institut Francais du Petrole, and U.S. Pat. No. 5,689,032, to Krause/Pasadyn. Other references that describe membrane-based separation of hydrogen from gas streams in a general way include U.S. Pat. No. 4,654,063, to Air Products, and U.S. Pat. No. 4,892,564, to Cooley.
The use of polymeric membranes to treat off-gas streams in refineries is also described in the following papers: xe2x80x9cHydrogen Purification with Cellulose Acetate Membranesxe2x80x9d, by H. Yamashiro et al., presented at the Europe-Japan Congress on Membranes and Membrane Processes, June 1984; xe2x80x9cPrisms(trademark) Separators Optimize Hydrocracker Hydrogenxe2x80x9d, by W. A. Bollinger et al., presented at the AIChE 1983 Summer National Meeting, August 1983; xe2x80x9cPlant Uses Membrane Separationxe2x80x9d, by H. Yamashiro et al., in Hydrocarbon Processing, February 1985; and xe2x80x9cOptimizing Hydrocracker Hydrogenxe2x80x9d, by W. A. Bollinger et al., in Chemical Engineering Progress, May 1984. These papers describe system designs using cellulose acetate or similar membranes that permeate hydrogen and reject hydrocarbons. The use of membranes in refinery separations is also mentioned in xe2x80x9cHydrogen Technologies to Meet Refiners"" Future Needsxe2x80x9d, by J. M. Abrardo et al. in Hydrocarbon Processing, February 1995. A chapter in xe2x80x9cPolymeric Gas Separation Membranesxe2x80x9d, D. R. Paul et al. (Eds.) entitled xe2x80x9cCommercial and Practical Aspects of Gas Separation Membranesxe2x80x9d, by Jay Henis describes various membrane-based hydrogen separations.
Besides individual treatment by PSA or membranes, numerous processes are known in which membrane separation and PSA are combined in a complementary way to carry out an integrated process. These include the following U.S. Pat. Nos. 4,229,188; 4,238,204; 4,398,926; 4,690,695; 4,701,187; 4,783,203; 4,836,833; 4,863,492, and 5,411,721.
In all of the above-cited references, the membranes used to perform the hydrogen/hydrocarbon separation are hydrogen-selective, that is, they permeate hydrogen preferentially over hydrocarbons and all other gases in the mix. A difficulty that hampers the use of both PSA systems and membrane separation systems of this type is the presence in off-gases of the C5 and heavier hydrocarbons, water vapor and hydrogen sulfide. In the case of membrane systems, the presence of these materials can cause catastrophic collapse of the membranes, as discussed in detail in co-owned U.S. Pat. No. 6,011,192, entitled xe2x80x9cMembrane-Based Conditioning for Adsorption System Feed Gasxe2x80x9d, which is incorporated herein by reference in its entirety. In the case of PSA systems, the C5+ hydrocarbons and other contaminants may sorb preferentially onto the bed, both reducing the capacity of the beds to sorb the light hydrocarbons that they are intended to remove, and giving rise to serious regeneration difficulties.
Instead of using hydrogen-selective membranes, it is possible to carry out membrane separations in which hydrocarbons permeate selectively and hydrogen is rejected in the residue stream. Processes that rely on selective permeation of hydrocarbons to separate at least some hydrocarbons from at least some other less condensable gases are taught, for example, in U.S. Pat. Nos. 4,857,078; 4,963,165; 5,032,148; 5,089,033; 5,199,962, 5,281,255; 5,401,300; 5,407,466; 5,407,467; and 5,501,722, all to Membrane Technology and Research (MTR). Literature from Membrane Associates Ltd., of Reading, England, shows and describes a design for pooling and downstream treating various refinery off-gases, including passing of the membrane permeate stream to subsequent treatment for LPG recovery. A report by Membrane Technology and Research, Inc. to the U.S. Department of Energy entitled xe2x80x9cLow Cost Hydrogen/Novel Membrane Technology for Hydrogen Separation from Synthesis Gasxe2x80x9d (October 1990) lists permeation data for polyamide copolymer membranes and shows diagrams indicating potential positions for membrane separation units in a coal gasifier train.
U.S. Pat. No. 4,857,078, to Watler/MTR, mentions that, in natural gas liquids recovery, streams that are enriched in hydrogen can be produced as retentate by a rubbery membrane.
An alternative approach, also using membranes that reject hydrogen and preferentially permeate hydrocarbons, is to use not a polymeric membrane but a carbon membrane, such as those taught in U.S. Pat. No. 5,104,425, to Air Products and Chemicals. These membranes are made up of a microporous adsorbent material on a porous substrate, and can separate gas mixtures based on selective adsorption onto the pore walls, rather than by the solution/diffusion mechanism of conventional polymeric membranes. Thus, the mechanism of separation is akin to the separation mechanism in PSA. This allows separation between various hydrocarbon fractions to be made, and hydrogen tends to be retained in the membrane residue stream.
It is known to combine these membranes with PSA to carry out integrated separations of light hydrocarbons from hydrogen. U.S. Pat. No. 5,332,424 describes fractionation of a gas stream containing C1-C4 hydrocarbons and hydrogen using a bank of membrane modules followed by a PSA unit. Other Air Products patents that show processes involving separation by carbon adsorbent membranes followed by PSA include U.S. Pat. No. 5,507,856 and 5,753,011. U.S. Pat. No. 5,435,836 teaches PSA followed by adsorbent carbon membranes for a similar separation. U.S. Pat. No. 5,634,354 teaches combinations of adsorbent membranes and PSA to treat gases containing hydrogen and olefins.
Adsorbent membrane systems similar to those disclosed in the above patents are described in papers by M. B. Rao and S. Sirkar in Journal of Membrane Science (Vol. 85, 253-264 (1993)) and Gas Separation and Purification (Vol.7, No.4, 279-284 (1993)). Adsorbent membrane/PSA hybrid systems are described in some detail in reports by M. Anand and K. A. Ludwig to the U.S. Department of Energy (xe2x80x9cNovel Selective Surface Flow Membranes for the Recovery of Hydrogen from Waste Gas Streamsxe2x80x9d, Phase I (1995) and Phase II (1996) Final Reports under contract number DE-FC04-93AL94461), and in materials distributed at a U.S. Department of Energy, Office of Industrial Technology, exhibit in Washington, D.C. (xe2x80x9cScale-Up of Selective Surface Flow Membrane for Gas Separationxe2x80x9d, T. Nahieri et al., Air Products and Chemicals, 1996).
In all of the above references, the gas mixtures introduced into the adsorbent carbon membrane system are limited to those containing no heavier than C4 hydrocarbons. In fact, the references are explicit that a pretreatment system (temperature swing adsorption) is used to remove C5+ hydrocarbons, water vapor and hydrogen sulfide that might foul the membranes. Since the membranes rely on adsorption for their separation properties, they are vulnerable to the same problems as PSA systems, namely that the more readily is a component sorbed, the more difficult is it to desorb. These contaminants, once introduced into the membranes, block the sorption sites and prevent the membranes functioning for their intended purpose.
Turning from hydrogen separation to hydrogen manufacture, steam reforming of light hydrocarbons is widely used. Typical steam reforming reactions are as follows:
CH4+H2Oxe2x86x92CO+3H2
CH4+2H2Oxe2x86x92CO2+4H2
C2H6+4H2Oxe2x86x922CO2+7H2
C3H8+6H2Oxe2x86x923CO2+10H2
C4H10+8H2Oxe2x86x924CO2+13H2
C5H12+10H2Oxe2x86x925CO2+16H2
xe2x80x83C6H14+12H2Oxe2x86x926CO2+19H2
The raw gas that results from these reforming reactions is a mixture of at least hydrogen, carbon dioxide, carbon monoxide, methane, water, and sometimes other components, such as nitrogen and argon. In many cases, a shift reactor is used after the primary reformer to convert carbon monoxide to carbon dioxide by the water gas shift reaction:
CO+H2Oxe2x86x92CO2+H2
The gas mixture that results from these reactions is known as synthesis gas. To produce high-grade hydrogen from the synthesis gas, the hydrogen must be separated from the other gases in the mix. Pressure swing adsorption (PSA) is widely used for this step, and can produce a hydrogen product with a purity of at least 99.9%. The tail gas stream produced when the PSA beds are regenerated is usually burnt to provide heat for the steam reformer.
The source of hydrocarbons for steam reforming is most commonly natural gas. In principle, however, many other streams containing light hydrocarbons, including various light overhead streams from refining and petrochemical operations, may also be used as hydrocarbon feedstocks for steam reformers. Many of these streams, such as the light ends from hydrocrackers, hydrotreaters, catalytic reformers and catalytic crackers, already contain non-negligible amounts of hydrogen, or may even contain hydrogen as the major component. This hydrogen does not take part in the steam hydrocarbon reforming reactions, yet occupies reformer space capacity. It is, therefore, inefficient to produce hydrogen from such streams and, despite their potential value as a hydrogen manufacturing feedstock, they are often burnt as fuel.
A number of patents concern treatment of mixtures of hydrogen, carbon dioxide, carbon monoxide and methane from steam reformers. U.S. Pat. No. 4,836,833 describes a process for recovering discrete product streams of hydrogen and carbon monoxide from synthesis gas by a combination of carbon-dioxide-selective PSA with a combined PSA/membrane step for carbon monoxide/hydrogen separation. U.S. Pat. No. 5,073,356 also concerns the production of carbon monoxide and hydrogen by steam reforming, using a gas separation scheme including PSA, vacuum swing adsorption (VSA) and membranes. In U.S. Pat. No. 5,435,836, the gas mixture from the steam reformer is treated by PSA to recover a high purity hydrogen stream. The waste gas from the PSA unit is then treated by membrane separation using a carbon adsorbent membrane. The hydrogen-rich residue is returned to the PSA unit and the permeate gas from the membrane unit can optionally be used as fuel for the steam reformer. U.S. Pat. No. 5,753,010 discloses a process similar to that of U.S. Pat. No. 5,435,836, but in which the tail gas from the PSA unit is split into two fractions of unlike composition, which are treated separately in two discrete membrane steps.
U.S. Pat. No. 5,354,547 discloses in FIGS. 2 and 3 process designs for integrating steam reforming, adsorbent carbon membranes and PSA to produce a high-purity hydrogen product. In FIG. 3 a side-stream from the reformer feed is run across the permeate side of the membrane as a sweep gas before being introduced as feedstock into the reformer. This process configuration is also shown in U.S. Pat. No. 5,447,559.
Patent application Ser. No. 09/083,560, now co-owned U.S. Pat. No. 6,011,192, describes a process in which a rubbery polymeric membrane is used to condition a gas stream to remove heavy hydrocarbons before PSA treatment.
Patent application Ser. No. 09/273,207, now co-owned U.S. Pat. No. 6,350,371, describes the use of hydrogen-rejecting membranes to treat tail gas from a PSA unit used to recover hydrogen from gas generated during catalytic reforming.
The invention is an improved process and process train for hydrogen separation and production. The invention uses incoming gas streams containing hydrogen and light hydrocarbons, specifically at least one C1-C4 hydrocarbon and at least one C5-C8 hydrocarbon, such as off-gas streams from oil-refining operations and the like. An important aspect of the process is that it includes both recovery of hydrogen already in the stream by membrane separation and PSA, and production of additional hydrogen by steam reforming of the hydrocarbons. By steam reforming, we mean the production of a synthesis gas containing at least hydrogen and carbon oxides from a feed mix including a light hydrocarbon, typically methane, and steam.
The process involves using a membrane separation step to separate hydrocarbons and hydrogen in the incoming gas stream. The membrane used is a polymeric membrane selective for hydrocarbons over hydrogen, which creates a hydrogen-depleted, hydrocarbon-enriched permeate stream and a hydrocarbon-depleted, hydrogen-enriched residue stream. The membrane separation step serves several purposes. In general, the heavier the hydrocarbon, the faster will be the membrane permeation rate. Thus, any C5-C8 hydrocarbons present in the incoming gas will be removed into the permeate stream faster and more completely than the C1-C4 hydrocarbons. The result is that the residue stream from the membrane separation step contains much less C5+ hydrocarbon than was present in the incoming stream. This can be expressed as a reduction in the hydrocarbon dewpoint of the residue stream. Most preferably, the dewpoint of the residue stream is at least about 10xc2x0 C. lower, as measured at 200 psia, than the dewpoint at 200 psia of the incoming stream. This hydrogen-rich stream bypasses the steam reforming step and is passed to PSA treatment. Here the hydrogen is separated from other gases in the mix to produce a high-purity hydrogen product stream. By using the membrane separation step upstream of the PSA unit, the membrane separation step serves as a conditioning step to protect the PSA unit from exposure to contaminants that are difficult to desorb once they reach the beds.
The hydrocarbon-enriched, hydrogen-depleted permeate stream from the membrane separation step provides hydrocarbon feedstock to the steam reformer. By reducing the amount of hydrogen and increasing the amount of hydrocarbon passing through the steam reformer, the unit processing capacity of the reformer can be utilized more efficiently. Thus, streams that would previously have been too rich in hydrogen and too lean in light hydrocarbons to be attractive as steam reformer feedstocks may now be used for hydrogen manufacture. The syngas product stream is withdrawn from the reformer and passed to the PSA unit, where non-hydrogen components are adsorbed, leaving a high-purity hydrogen stream as product.
Thus, in its most simple form, the invention includes three unit operations or steps: the membrane separation step, the steam reforming step and the pressure swing adsorption (PSA) step. The membrane separation step divides the gas stream to be used in the process into a hydrogen-rich portion and a hydrocarbon-rich portion, and may be carried out in one or multiple stages or steps. The hydrocarbon/steam reforming reactions may be performed in any manner and using any types of reactors, catalysts and operating schemes known in the art. The reactor arrangement may, but need not necessarily, include a shift reactor downstream of the hydrocarbon reforming reactor, the purpose of which is to convert carbon monoxide formed in the reforming reactor to carbon dioxide. The hydrocarbon feedstock to the steam reformer comprises the membrane permeate stream. Frequently, but not necessarily, the permeate stream supplements, or is supplemented by, other hydrocarbon feedstock material, such as a natural gas stream. The PSA step may be carried out by any convenient manner known in the art and typically involves the use of a series of beds connected in such a way that each bed can be switched periodically from adsorption mode to regeneration mode. The tail gas produced when the PSA beds are regenerated may be burnt as fuel to heat the steam reformer. In the process of the invention, two streamsxe2x80x94the membrane residue stream and the synthesis gas streamxe2x80x94need to be treated by PSA. This can be done by feeding the streams to discrete PSA units, or by feeding them independently or together to the same unit.
In a basic embodiment, these treatment steps take the following form:
(a) providing a gas stream containing at least a C1-C4 hydrocarbon, a C5-C8 hydrocarbon and hydrogen;
(b) passing the gas stream across the feed side of a polymeric membrane having a feed side and a permeate side, the membrane being selective in favor of the C1-C4 hydrocarbon and the C5-C8 hydrocarbon over hydrogen, under conditions such that a driving force for transmembrane permeation is provided by a pressure difference between the feed and permeate sides;
(c) withdrawing from the permeate side a hydrocarbon stream enriched in the C1-C4 hydrocarbon and the C5-C8 hydrocarbon and depleted in hydrogen compared with the gas stream;
(d) withdrawing from the feed side a residue stream enriched in hydrogen compared with the gas stream;
(e) feeding the hydrocarbon stream to a steam reformer and there reacting the C1-C4 hydrocarbon and the C5-C8 hydrocarbon with steam to form a synthesis gas stream;
(f) passing the synthesis gas stream and the residue stream together or separately through a pressure swing adsorption system capable of selectively adsorbing hydrocarbons and rejecting hydrogen;
(g) withdrawing a purified hydrogen product stream from the pressure swing adsorption system.
In another aspect, the invention is an apparatus comprising a specific combination of a membrane separation unit capable of producing a hydrogen-enriched residue stream and a hydrocarbon-enriched permeate stream, a steam reformer, and a PSA unit capable of selectively removing hydrocarbons from hydrogen.
The invention has a number of advantages. In particular, the process yields more high-purity hydrogen from the same amount of raw hydrocarbon feedstock or from the same steam reformer reactor capacity than is possible using prior art processes.
The invention differs from the numerous prior art combinations of membrane separation with PSA of which applicants are aware in several regards. First, the membrane separation and PSA steps are integrated with the steam reforming step in such a way that raw gas entering the process is treated first by the membrane separation step. Furthermore, both the membrane residue and permeate streams pass through the PSA step, (although the permeate stream has by then been significantly changed in composition in the reforming step).
Yet another difference and advantage is that the membrane separation step tolerates exposure to heavier hydrocarbons and indeed protects the adsorption system by removing these components from the feed gas. As was discussed above, if C5-C8 hydrocarbons, or even heavier hydrocarbons, reach the adsorbent system, they sorb very readily onto the beds. Bed regeneration is typically carried out by lowering the pressure on the bed, thereby desorbing the previously sorbed materials and flushing them out of the bed. Since C5-C8 hydrocarbon components are liquid at room temperature and pressure, they are difficult to desorb, and tend to remain in the bed, causing progressive fouling. To remove such contaminants, it may even be necessary to draw a vacuum on the bed, which increases the cost and complexity of operation substantially. These problems are ameliorated or avoided completely by the upstream membrane step.
Since the membrane can withstand heavier hydrocarbons, pretreatment steps before the membrane separation step to remove them, although optional, are not necessary. This contrasts with cellulose acetate and like membranes, which can suffer catastrophic failure if hydrocarbons condense within the membrane modules. Also, unlike other types of hydrogen-rejecting membranes, such as adsorbent carbon membranes, the presence of a heavier hydrocarbon component does not have a significant negative impact on the permeation of a lighter component. For example, the presence of small amounts of C8 and above hydrocarbons will not impede the ability of the membrane to remove C6 components. Thus, the membranes can handle a diversity of stream types that would be impossible to treat in prior art processes.
Another benefit is that polymeric materials are used for the membranes. This renders the membranes easy and inexpensive to prepare, and to house in modules, by conventional industrial techniques, unlike other types of hydrogen-rejecting membranes, such as finely microporous inorganic membranes, including adsorbent carbon membranes, pyrolysed carbon membranes and ceramic membranes, which are very difficult and costly to fabricate in industrially useful quantities.
Unlike many other polymeric membrane separation processes that have been used to separate hydrogen from hydrocarbons in the past, the present process uses membranes that are hydrogen-rejecting. That is, the hydrocarbons permeate the membrane faster than hydrogen, leaving a residue stream on the feed side that is concentrated in the slower-permeating hydrogen. This means that the stream may be passed to the PSA step without recompression. This provides an advantage compared with the use of hydrogen-selective membranes, which produce permeate hydrogen streams at low pressure. Such stream require significant compression before being sent to PSA.
A pressure difference between the feed and permeate sides provides the driving force for transmembrane permeation in the membrane separation step. If the gas to be treated is already at elevated pressure, it may be passed directly to the membrane separation step. Otherwise the gas is compressed before passing to the membrane unit. Optionally, for example if the stream is comparatively rich in C5+ hydrocarbons, both a compression step and a cooling step can be carried out upstream of the membrane unit, to enable the C5-C8 hydrocarbons to be recovered from the process in liquid form as a separate product.
Specific exemplary streams to which the process of the invention can be applied include, but are not limited to, off-gas streams from hydrocrackers; hydrotreaters of various kinds, including hydrodesulfurization units; coking reactors; catalytic reformers; catalytic crackers; specific isomerization, alkylation and dealkylation units; and hydrogenation and dehydrogenation processes. The invention can be applied to any streams containing hydrogen, a C1-C4 hydrocarbon, and a C5-C8 hydrocarbon. The presence of the C5-C8 hydrocarbon component means that most streams for which the invention is useful are characterized by a hydrocarbon dewpoint at 400 psia of at least about 10xc2x0 C., and many are characterized by a hydrocarbon dewpoint at 200 psia of at least about 10xc2x0 C. This does not mean that the gas is at 400 psia or 200 psia before, during or after treatment (although it could be), but merely serves to express the hydrocarbon content of the gas in a definite way. Many gas streams to be treated by the invention have higher dewpoints, such as 20xc2x0 C., 30xc2x0 C., 40xc2x0 C. or 50xc2x0 C., all as measured at 200 psia.
The invention is especially useful for treating streams that are neither very rich in heavier hydrocarbons nor very rich in hydrogen. By this, we mean streams that contain no more than about 80% hydrogen and no more than about 10% C5-C8 hydrocarbon. Absent the process of the invention, such streams are typically used as fuel gas. The invention provides separation and recovery of the valuable hydrogen already in the stream, and efficient use of the hydrocarbons in the stream to make more hydrogen.
Furthermore, the invention reduces the fuel gas load in the plant, by utilizing streams that would previously have been sent to the fuel gas header. In plants where fuel gas generation is at capacity, the invention provides debottlenecking capability, allowing throughput of the unit operations generating the off-gas to be increased.
It is to be understood that the above summary and the following detailed description are intended to explain and illustrate the invention without restricting its scope.